To win the game, the coin would need to land on heads two out of three times. Kyle spun the quarter like a silver top. First roll, heads. Second roll, tails. On the third roll, the coin broke its tight spiral and teetered left and right like a seesaw.

Finally, it stopped. Heads. Kyle had survived the first round. "The way you win is with good cards," says Kyle, a bright-looking boy with closely cropped hair.

Now all Kyle had to do was find the perfect card to play - for three more rounds after this.

Here in the Liberty Tree Mall in Danvers, Mass., north of Boston, more than 100 children and teenagers had gathered to play in what is quickly becoming a major event in kid culture: Yu-Gi-Oh trading-card tournaments.

Each week, mostly on Saturdays, kids gather in malls like this one, or in stores that sell board games and magic tricks, to play Yu-Gi-Oh.

The cards, sold by Upper Deck (of baseball-card fame), first came to the United States about a year ago. The game has gotten hugely popular.

But trading cards are only part of Yu-Gi-Oh. A lot of you reading this already know about it because of the Saturday morning cartoon or the video game. You might also have seen a picture of Yu-Gi, the cartoon's main character, on T-shirts, key chains, and puzzles.

You may get the feeling that you've seen ideas like this before. Remember Pokémon? That also was a trading-card game, a cartoon, and a toy.

What about Star Wars? It started as a movie, but now it includes card games and merchandise. Even books, like the Harry Potter series, follow this pattern.

One idea, many many products

For the past 10 years, companies that come up with a cool idea like Yu-Gi-Oh have made a lot of money by using that idea in many different products, TV shows, and movies for kids.

By spreading the idea around, companies make it seem impossible for kids not to buy something related to the fad.

There's nothing wrong with buying these products. But kid-entertainment experts say kids should be more aware of how companies get them interested in products. Kids also need to know why companies make these products.

"It's about getting you to spend your allowance money on these things," says Kathryn Montgomery, president of the Center for Media Education in Washington, D.C.

The Yu-Gi-Oh brand is a good example of how a good idea becomes a fad.

Yu-Gi-Oh began as a comic book in Japan in 1996. The character Yu-Gi is a kid in high school whose grandfather owns a game store. The grandfather gives Yu-Gi an ancient Egyptian puzzle.

When Yu-Gi solves the puzzle he receives magic powers. One of his talents is playing Dueling Monsters - a card game in which two players battle each other using cards based on mystical creatures with different abilities.

The comic book was a huge hit in Japan, partly because it involved trading cards, which many kids were playing.

The company that owns the Yu-Gi-Oh idea in Japan has made more than $2 billion. Because other products that did well in Japan have become big successes in the US (Power Rangers, "virtual pets," Nintendo, etc.) the company sold the idea to the same US company that produced Pokémon - 4Kids Entertainment.

Even before the cartoon went on TV in the fall of 2001, 4Kids was slowly introducing it. "We had to tell kids who [Yu-Gi] was, how the game was played, what various power levels the monsters had, and who his circle of friends were," says Steve Syatt, a 4Kids spokesman.

The company did not want kids to get turned off by a lot of advertising. So first they asked a lot of websites that review new games for kids, like yahooligans.com and figures.com, to write stories about it.

Sending out 1 million free videos

The company then mailed more than 1 million videos with information and scenes from the cartoon to kids across the country. 4Kids got the list of kids' names and addresses from Toys 'R Us.

4Kids had done the same thing years ago to stir interest in Pokémon. "It worked so incredibly well the first time," Mr. Syatt says. "[We wanted to] get the word out directly to kids."

The cartoon began airing on TV in the fall of 2001. Soon after, Yu-Gi-Oh video games and action figures started showing up in toy stores. But when the trading-card game came out last spring, Yu-Gi-Oh took off. Many kids, like Kyle Dickson, also at the tournament here, learned the game at summer camp.

What attracted Kyle to the game? "The powers are interesting," says the 10-year-old from Beverly, Mass. "I feel like getting all the cards that exist."

Many who play the game are like Kyle. They want to get all the cards so they can play the best game possible. The rarest cards have the best powers.

The companies that sell the cards know this. That's why they design them so that the kids who are most interested in the game have to spend lots of money.

A nine-card "booster pack" of Yu-Gi-Oh cards costs about $3. Each booster pack includes one "rare card." But ultra-rare cards, which are even more powerful, are in only one of every 12 packs.

To get the best cards, you have to buy lots of them. Most of the cards you end up with will be ones you already have.

"They want to keep you hooked," says Douglas Gentile, director of research at the National Institute on Media and the Family, in Minneapolis. "Their goal is to get you to spend more money, and to get you to get your friends to spend more."

Are cheaper games just as fun?

Instead of getting interested in a game that requires a lot of money to play better, many toy experts recommend that you look for games that only require one purchase, but can still play out in hundreds of different ways.

"Yu-Gi-Oh requires strategic thinking," says Stevanne Auerbach, a play and toy expert in San Francisco, "but games like chess and checkers have unlimited strategies and can be more interesting and challenging."

Still, 4Kids is trying to keep Yu-Gi-Oh from becoming just another fad that fades. The company has decided to allow only a limited number of other companies to use the Yu-Gi-Oh name and characters to sell products.

"We want it to be here for the long haul," says Syatt, who says 4Kids wants to maintain Yu-Gi-Oh's "underground" reputation. 4Kids also wants to make sure that older kids and teenagers - between the ages of 9 and 14 - keep buying Yu-Gi-Oh products. The company will not advertise Yu-Gi-Oh to younger kids. When Yu-Gi-Oh appeals to little kids, "that's the day the big kids will [no longer be interested]," Syatt says.

Regardless of how well he does at Yu-Gi-Oh tournaments, Kyle Amirault says he'll play for a few more years. "I'll play as long as it's fun," he says.

The card game that started it all

Walk into most toy stores nowadays, and trading-card games are easy to find. If a new adventure movie for kids is in theaters, count on a new trading-card game in stores soon.

But that wasn't always the case. Just 10 years ago, trading-card games didn't even exist!

The origin of the trading-card industry can be traced to "Magic: The Gathering." Magic was the first game in which cards with characters having different powers face-off against one another.

Since its debut in 1993, Magic has gotten very popular. Its producer, Wizards of the Coast (in Renton, Wash.), has created 6,000 kinds of cards. More than 7 million people have played Magic at more than 100,000 official tournaments.

Richard Garfield, the Seattle-based game consultant who invented Magic, got the idea by fiddling with games like chess and Monopoly.

"At a certain point," Mr. Garfield says in an e-mail interview, "it hit me that the modification of the game could be the game itself. From there it is a short leap to a game where each person chooses which cards he is going to play from a vast pool of cards - creating, in a sense, their own game."

In traditional games, Garfield says, the action is limited by what is in the cards and on the board. He says trading-card games generally (and Magic in particular, of course) have attracted millions of fans because the game can take many paths.

"Players of trading card games enjoy the boundlessness of the experience," Garfield says. "Each player has a unique part of the game, so that each player you play brings you surprises and teaches you more."

Unlike many trading-card games, success in Magic does not require a player to buy expensive rare cards. Common cards, Garfield says, have a lot of power. Rare cards are more complex and unusual, but not necessarily more powerful.

But when it comes to cost, parents and kids should consider the value of trading-card games, Garfield says. He considers the games to be a "mental sport," so to him it's OK if families spend as much money on these games as they do on sports.

Garfield's goal, he says, was to make a great game, not just something to get people interested in other products.

"Magic is not driven by books, or movies, or TV, or video games," Garfield says. "It is my hope that Magic is, like a very few games such as Scrabble or chess, timeless." (But if he could do it all over, he says, he would have been interested in having a Magic TV show and film.)

Yes, Garfield plays Magic, among many other games. "I like board games, card games, computer games ... traditional games, role-playing games, physical games (like Jenga and darts), party games," he says. "I am sure there are other categories I am missing, but you get the picture."